1. Field of the Invention
The present invention relates to a receiver circuit, a reception method, and a communication system. More particularly, the present invention relates to a receiver circuit, a reception method, and a communication system that compensates demodulation characteristics of an analog filter using a digital filter.
2. Description of Related Art
With recent enhancement of high-speed data transfer capability such as HSDPA, mobile communication terminal devices for wireless CDMA and the like are now required to improve modulation accuracy, that is, to limit modulation accuracy (Error Vector Magnitude, hereinafter referred to as EVM) of a reception section of a radio below several percent.
To improve EVM, receiver circuits that combine an analog filter and a digital filter have been proposed (see Japanese Patent Application Laid-Open Publication No. 2000-269785, for instance). The circuit described in the publication removes most part of an interference signal which has been taken in superimposed on a desired received signal or an interference signal far from a carrier with an analog filter, and removes remaining interference signal components with a digital filter. Thus, by using an analog filter and a digital filter in combination, an interference signal can be efficiently removed and a desired received signal can be retrieved.
When an analog filter and a digital filter are used in combination, the digital filter is often given a function of compensating imperfection in in-band characteristics caused in the analog filter, e.g., gain ripple or phase rotation due to group delay variation. Distribution of characteristics between an analog filter and a digital filter is defined when functional characteristics of the filters are designed in consideration of various conditions, such as a dynamic range of an A/D converter, EVM required for the radio, and input specifications of an interference signal.
However, because a digital filter responsible for not only removal of an interference signal but compensation of analog filter characteristics is composed of a finite impulse filter (hereinafter referred to as an FIR filter), the FIR filter has a longer tap length (i.e., a length of a tap coefficient) and/or a longer tap width (accuracy of a tap coefficient) than those of a typical digital filter (a root Nyquist filter). Since an FIR filter requires a larger hardware and consumes greater electric power as its tap length or width becomes greater, an FIR filter has a problem of hampering reduction of power consumption of a mobile communication terminal device.